1. Field of the Invention
The present invention is generally related to an optical pickup for reading information contained on an optical data storage media, and more particularly to an optical pickup apparatus and method utilizing multiple read beams to reduce effective read beam spot size.
2. Description of the Background Art
Optical data storage provides an inexpensive and high bit density, non volatile data storage method. Optical media is typically configured as an information plane within a record carrier that is configured for rotation wherein a plurality of tracks at fixed radial distances from the center of rotation are described, or a single track is configured as a spiral in a similar manner as found on vinyl records in previous eras. Data is read from the media by directing a beam of light onto the media and detecting the reflection of the beam from the media with a detector. Optical features of the media are then discerned from the electrical signal generated by the detector as data bits encoded within the media are reflected as features within the waveform of the electrical signal. It will be appreciated that the duration of the waveform feature is proportional to the beam diameter used to scan the surface of the media, therefore, larger beam diameters result in extended waveform feature lengths. Although holographic and other forms of data encoding are available, data is typically encoded onto the surface, or reflective subsurface, of the media in the form of pits. The minimum size of each pit is largely determined by the diameter of the beam of light that impinges on the surface of the media for reading the data.
The density of optical data storage has continued to increase as the wavelengths of the laser light sources used shrinks. It will be appreciated that the spot diameter of a beam of light for reading pit information will be at or greater than the wavelength lambda (λ) of the light source. The spot diameter of the laser beam is proportional to the wavelength lambda (λ), and is inversely proportional to the numerical aperture (NA) of the objective lens. In most high density optical media information is read using a laser light source of a short wavelength approaching the pit diameter and an objective lens having a large numerical aperture (NA).
It will be appreciated that the cost of a laser source is highly dependent on the wavelength of the light generated, with shorter wavelengths being more costly than longer wavelengths of light. Increasing the storage density of the optical media, therefore, results in cost increases from the need for higher resolution head positioning and for shorter wavelength light sources. In addition, in select applications the beam size must be adjusted to accommodate media which has been encoded at different densities, such as for example, DVD optical disks and CD disks. Presently, optical drives that read multiple densities rely on mechanical means for matching the spot size to match the media, such as changing of the lens optics, wherein a shorter wavelength of light may be utilized to read either smaller or larger pit sizes. The current reliance on matching the optical properties of the beam with the size of the pits encoded within the media, increases the cost of optical data storage devices and limits the bit densities that are economically available.
Therefore, a need exists for a method and apparatus that provide for the reading of high density optical media without the need of more expensive short wavelength light sources as outlined above, and which can be adjusted for reading optical media having different densities. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed optical pickups.